Add distrib doc

[ybertot]

[ybertot]

The lack of documentation of the release process is impeding collaboration, expected with respect coq-platform (see https://github.com/coq/opam-coq-archive/pull/2080#discussion_r792081189 ). The file DISTRIBUTION.md attempts to clarify the situation.

This is human-readable prose, there are practically no ways to check its correctness.

Ultimately, it would be great to limit the manual process to explaining what version of coq is targeted, and then let the build process rely on the exact content of the git repository at a specific date in the coq-master branch, which would only need tagging.